1. Field of the Invention
The present invention relates to a device for generating acoustic waves by means of a falling mass striking a target element coupled to the wall of a well or drill hole, usable in particular in the field of land based seismic prospection.
2. Description of the Prior Art
A device is known for generating seismic impulses by means of a mass striking a target element anchored in a drill hole. The target element is secured to an elongate body which is provided with anchorage means.
The mass is movable in the body between a low position in contact with the target element and a high position from which its fall is initiated by the action of control means. These means include an element with retractable hooks capable of gripping an engagement piece integral with the mass and rigid traction elements (rods or string of interconnected rods) connected to lifting means disposed outside the well. The anchorage means are of the "packer" type well known to specialists and include an expandable member having a radially movable peripheral part and a central part.
Expansion is obtained by rotating the two parts with respect to each other by rotating the string of rods from the surface. The peripheral part may be formed of pivoting arms provided with claws which, by opening, anchor themselves in the wall of the well, or else of a deformable enclosure. Considering the method of operation of the device, the radial stresses are exerted alternately upwardly and downwardly on the anchorage means.
Upwardly, when the mass is raised by pulling the string of rods, because of the friction and the hydrostatic pressure reigning in the well. Downwardly, at the moment when the mass impacts the target element. With the anchorage means of the "packer" type used in practice, the body is immobilized by jamming anchorage arms against the wall of the well and the positions of the anchorage means where this occurs are not in general identical dependent on whether the body is being lowered or raised. The result is that the assembly is subjected to a shift during the same operating cycle, which causes damage to the walls and reduces the coupling coefficient.
Furthermore, the acoustic power emitted by this source depends on the falling height of the mass. For some applications in which the acoustic power required is high, a falling height of several meters may be required. The total height of the device may then be very great, which sometimes makes handling of it difficult.
The prior device is described in the French patent application EN. No. 85/17 832.